A closed breeding colony of Virginia opossums (Didelphis marsupialis virginiania) is maintained to provide neonates for research purposes. Opossums are bred during the 4 month breeding season (January-April) by establishing natural matings within a group of 2-3 females to 1 male. Mortality is high among males who are killed by the females. The conception rate in 1976 was 22% (13 litters from 49 matings) as compared to 43% in 1975. This continual decline in conception over the past several years suggests that subtle environmental and/or husbandry factors adversely affect fertility of captive born-reared opossums. The colony was discontinued in November, 1976 and the animals were shipped to Germany to establish breeding colonies.